1. Technical Field
This invention relates to cable television systems and, more particularly, a method and apparatus for applying remotely controlled and remotely applied interdiction or jamming signals to prevent reception of unauthorized television channels.
2. Description of the Prior Art
At a headend of a cable television system, a scrambler is normally provided to encode premium television channels. The applied scrambling precludes reception by an unauthorized converter/decoder at a connected premises. Data representing channels or tiers of programming are addressably transmitted to a particular converter/decoder and stored in an authorization memory. As a result of the addressed transmission, a subsequently transmitted program is authorized in that the decoder portion of the converter/decoder will be selectively enabled to decode the scrambled premium channel or program.
Several varieties of scrambling techniques are applied today. Each manufacturer has its own scheme which may be incompatible with others. Nevertheless, most popular scrambling systems today are based on sync suppression, in which the sync information is hidden from the television receiver's sync separator, usually by moving it to a level occupied by picture information (moving the sync tip to an equivalent picture level of 40 IRE units is common). Some systems modulate the picture carrier with a sine wave phased to suppress the horizontal blanking interval. Most systems today switch to the suppressed level at the beginning of the blanking interval and switch out at the end. Most though not all suppress the vertical blanking interval. Some systems dynamically invert the video, either on a line-by-line or a field-by-field basis. This must be done carefully to avoid artifacts caused by inverting and reinverting around different levels, and by differential gain and phase of the system. Synchronization is restored either by the provision of synchronous amplitude modulated pulses on the sound carrier, by digital information placed in the vertical interval or by phase modulation on the picture carrier.
The provision of one scrambler per premium channel at the headend and the inclusion of a descrambler in each converter/decoder at the premises of the television receiver is particularly expensive. Furthermore, providing the converter/decoder on premises has turned out to be a great temptation to service pirates who imaginatively seek ways to receive premium channels. As a result, cable television equipment manufacturers have entered into a vertiable war with such pirates resulting in complicated service authorization protocols in some instances involving multiple layers of encryption by both in-band and out-of-band data transmission further increasing the costs of the converter/decoder.
Furthermore, all scrambling systems leave artifacts in the horizontal blanking interval in the form of steps on the front and back porches. Normally these are not a problem, but if a television receiver does not have adequate overscan, then the steps can show up as light bars on one or both sides of the picture. Further, if a television receiver uses back porch sampling for automatic gain control and/or black level restoration, and the sampling period extends into the time of the descrambling step, the television will show the wrong black level and may show flicker in the picture. In systems in which pulse trains are applied to the sound carrier, a buzz carried by harmonics of a 59.94 Hz signal can be noticed in some television receivers.
Consequently, the cable industry has resorted to look for new technology and to take a second look at technology developed in the early stages of development of cable television such as the application of negative and positive traps and more recent techniques such as interdiction.
Negative trap technology is viewed by many manufactures as a viable alternative to sync suppression scrambling methods. A negative trap is basically a narrow band reject filter. Traps are located at the drop to a subscriber's dwelling and attenuate a significant portion of a premium television channel rendering that channel unusable by the subscriber.
In the conventional embodiment, negative traps are made using L-C filter techniques. The result is a notch with finite quality Q and finite shape factor. In the case of a single channel negative trap, the center of the notch is usually located at the picture carrier frequency of the channel to be removed. This technique, sometimes called a static negative trap, requires attenuation at the picture carrier of at least 60 dB to be effective.
Negative trap systems have several advantages that make them attractive for cable television applications. One primary advantage is the ability to deliver a broadband cable television spectrum to the subscriber's converter/decoder. Conventional sync suppression systems utilize descrambling set-top converter/decoders which deliver inherently narrowband signals. Negative traps are usually mounted outside the subscriber's home (typically at the tap) and thereby minimize the exposure associated with placing hardware inside the subscriber's dwelling. Finally, some cable television operators view the negative trap as a more secure means of subscriber control than is sync suppression, as picture reconstruction is viewed as substantially more difficult.
However, the negative trap system requires hardware in locations where no revenue is generated for the cable television system. Moreover, negative traps have several severe practical limitations. L-C band reject filters have Q and shape factor limitations. Quality factors Q for L-C filters are typically limited to less than 30. This means that for a negative trap located at channel 8 (picture carrier at 181.25 MHz) the 3 dB bandwidth of a negative trap is typically 6 MHz (or the bandwidth of a baseband television channel). This trap would result in significant deterioration of the lower adjacent channel. Then the television receiver tuned to the lower adjacent channel, rather than having to contend with a 15 dB picture-to-sound ratio, may have to contend with a sound carrier reduced an additional 6 dB or so. Frequency stability as a function of time and temperature is also a significant concern. Many cable television system operators have instituted a regular negative trap change-out program based on the assumption that after a certain period of time and temperature cycling, frequency drift will render negative traps useless.
Cascadability is another significant concern. Finite return loss and non-zero insertion loss limit the number of single channel negative traps which can be cascaded. As the number of services to be secured increases, the negative trap decreases in appeal. Moreover, a change in a channel line-up requires a significant investment in hardware and manpower in this scenario.
Recently, a new type of negative trap has been introduced. The dynamic negative trap consists of a notch filter that is designed to be modulated with respect to frequency. The notch is centered about the picture carrier but is deviated slightly from side to side. The television channel is rendered unuseable by the introduction of unwanted amplitude and phase modulation on the picture carrier. This technique requires a notch depth significantly less than that of static negative traps (typically 40 dB). Additionally, the intentionally introduced frequency modulation reduces somewhat the requirement for frequency stability.
The dynamic negative trap, however, has several disadvantages. A power source is required in order to accomplish the frequency modulation. More significant is the parasitic modulation that this technique produces on the adjacent television channels.
Positive trap systems also utilize a narrow band-rejection notch filter. However, unlike negative trap systems which are used to attenuate or trap a premium channel transmission, the notch filter is used to restore the premium television channel. In this scenario, an interfering signal is placed inside the premium television channel at the cable television headend. This interfering signal is then removed at the subscriber's dwelling by use of the notch filter. Ideally this notch filter removes only the interference without removing a significant amount of television information.
The positive trap technique is seen as having several advantages by the cable television system operator. It is considered advantageous to have the interference present in the secured channels on the cable television distribution plant (unlike the negative trap system in which the channels to be secured are "in the clear" on the distribution plant). It is very attractive from a financial standpoint to require subscriber hardware only at those locations where a subscriber wishes to receive a secure service. Thus, any capital investment is associated with a point of revenue generation.
The conventional embodiment of the positive trap system utilizes L-C notch filters to remove the interfering signal. These L-C notch filters suffer from the same limitations as do L-C negative traps discussed above. Consequently, L-C based positive traps are limited to the lower end of the cable television spectrum. Quality Q and shape factors have also restricted the number of locations for the interfering signal within the television channel.
The location for the interfering signal in the conventional embodiment of the positive trap system is midway between the picture carrier and sound carrier. The energy density (and hence information density) in this area of the spectrum is relatively low. One reason this location was chosen was that it minimized the impact of any television information removed along with the interfering signal by the notch filter, and thereby improved the quality of the recovered television signal. It would be expected that the jamming carrier would normally have minimal effect on the adjacent channel television picture unless a television has unusually poor rejection 2.25 MHz above the picture carrier. The jammer does add another carrier which the tuner will have to contend with, which might cause some degradation in a marginally overloaded case.
Despite this location, the quality Q and shape factor limitations of conventional L-C positive traps do remove a significant amount of useful television information. The result is a noticeable "softening" of the television picture as a result of attenuation of high frequency information. Predistortion at the headend can improve this performance but falls far short of being able to correct it completely. This location for the interfering signal also facilitates the job of the video pirate. This pirate can easily tolerate a degraded signal and hence can recover a useable picture using techniques easily available (such as the classic twin lead quarter wave stub with an aluminum foil slider for the fine tuning). Also, positive trap systems require a higher per premium channel cost than a negative trap system.
Parallel to developments of different types of trapping or jamming systems, the cable industry has also evidenced a requirement to move a converter or descrambler outside of a subscriber's home to a location which is more secure from signal piracy. For example, an addressable tap system was developed by Scientific Atlanta in 1983 or 1984 in which an off-premises "tap", addressed by a headband control system, gates a premium channel into the subscriber's premises. However, such products did not prove to be viable alternatives to inside-the-home signal descrambler/converters.
A relatively recent technique for premium channel control is the interdiction system, so-called because of the introduction of an interfering signal at the subscriber's location. Most embodiments consist of a pole-mounted enclosure located outside the subscriber's premises designed to serve four or more subscribers. This enclosure contains at least one microprocessor controlled oscillator and switch control electronics to secure several television channels. Control is accomplished by injecting an interfering or jamming signal into unauthorized channels from this pole-mounted enclosure.
For efficiency's sake, it is known to utilize one oscillator to jam several premium television channels. This technique not only reduces the amount of hardware required, but also maximizes the system flexibility. The oscillator output jamming signal frequency is periodically moved from channel to channel. Consequently, the oscillator is frequency agile and hops from jamming one premium channel frequency to the next.
One such system is known from U.S. Pat. No. 4,450,481 in which a single frequency agile oscillator provides a hopping gain-controlled jamming signal output to four high frequency electronic switches. In this known system, each switch is associated with one subscriber drop. Under microprocessor control and depending on which subscribers are authorized to receive transmitted premium programming, the micro-processor selectively gates the jamming signal output of the single oscillator via the switches into the path of the incoming broadband television signal to each subscriber. Consequently, an unauthorized subscriber upon tuning to a premium channel will receive the premium channel on which a jamming signal at approximately the same frequency has been superimposed.
In the known system, it is indicated that sixteen channels may be jammed by a single voltage controlled frequency agile oscillator. With respect to one premium channel, this translates to a situation in which the jamming signal can only be present one sixteenth of the time or an approximately 6% jamming interval. The rate of hopping is also indicated at 100 bursts per second of jamming signal at a particular frequency or a 100 hertz hopping rate. Consequently, the effectiveness of the jamming signal is questionable.
Cable television channels and, of course, premium service may extend over a wide range of frequencies, for example, from 100 to 350 megahertz. In the known system, the single oscillator provided must be frequency agile over a wide range. It is further recognized that the jamming signal output of the single oscillator must be within a range of 100-500 KHz above or below the video carrier frequency. Consequently, a synthesizer having an internal reference is provided to assure the reasonable accuracy of the jamming signal output of the oscillator to a tolerable 100-500 KHz band above or below the video carrier.
It is indicated that the jamming signal is at a high relative power and is gain controlled to exceed the amplitude of the video carrier by 5 to 20 dB. Because of the high output power relative to the premium channel video carrier power and the difficulty of precisely jamming the premium channel frequency, such an interdiction system leaves considerable opportunity for improvement. Because the oscillator is frequency hopping, its spectrum tends to spread out around the picture carrier, generating a slightly different situation as far as the required adjacent channel rejection characteristics of the television signal are concerned.
Jamming oscillators usually operate near the picture carrier frequency of the television signal and preferably at an amplitude near the amplitude of the television signal. Should the amplitude of a jamming oscillator be too low with respect to the amplitude of the picture carrier, inadequate jamming of the channel may occur resulting in a recoverable picture by the subscriber. On the other hand, should the amplitude of a jamming oscillator be too high with respect to the amplitude of the picture carrier, artifacts may be generated in unsecured adjacent television channels. Such is also the case when the frequency of a jamming oscillator varies from the picture carrier frequency of a channel to be jammed.
It is important that an interdiction system jamming signal frequency be placed as close as possible to the picture carrier frequency. Otherwise, adjacent channel artifacts or incomplete jamming will result. In the known system, the jamming signal is intentionally placed below the video carrier and consequently approximate to an adjacent channel producing adjacent channel artifacts.
It is also important that a variable frequency oscillator in an interdiction system hop between frequencies quickly and accurately with little harmonic frequency effects. Otherwise, adjacent channel artifacts or incomplete jamming will result. Furthermore, by using only one jamming oscillator only a limited, small number of channels may be jammed. The known system uses a conventional voltage controlled oscillator controlled by conventional frequency control techniques. Furthermore, in the known system, a maximum six percent jamming interval results when sixteen premium channels are jammed by the single oscillator at a relatively slow rate of frequency hopping. In such a system, the result depth of jamming for an unauthorized premium channel is unsatisfactory.
Additionally, it is important in an interdiction system that the jamming signal be properly matched in gain with the level of an interdicted channel. Furthermore, it is important that gain of a jamming oscillator match the level of an interdicted channel not only to compensate for drifts in the components due to temperature variations and seasonal weather changes but to compensate for level variations due to its location in a television distribution plant and to compensate for tilt due to imperfect gain requirements of a distribution cable over the frequency spectrum. Otherwise, adjacent channel artifacts or incomplete jamming will result. In the known system, conventional gain sensing and control circuits are used for gain control to compensate only for the simplest of variations.
Consequently, prior to the present invention, the need remained for an interdiction system capable of accurately jamming a broad spectrum of frequencies such that a large number channels may be jammed in a cost effective manner without the problems of harmonic effects, adjacent channel artifacts or incomplete jamming. The present invention solves the above-mentioned problems and provides an interdiction system with improved jamming oscillator designs, improved gain control techniques and improved frequency control techniques not heretofore possible.